Germanium and doped germanium have widespread uses in electronics, photodetectors, and optics. Some of the well established properties of germanium which are of interest to the aforementioned fields of uses are set forth below.
Germanium is a metallic element, atomic weight 72.59, atomic number 32, and a member of the carbon family. Germanium is a grayish-white metal; a semiconductor of electricity, whose conductivity depends largely on impurities present. Other properties include specific gravity of 5.323, melting point 937.4.degree. C., boiling point 2830.degree. C.; does not volatilize below 1350.degree. C.; hardness 6 on Mohs scale.
Germanium is derived or recovered from residues resulting from the refinement of zinc and other sources, by heating in the presence of air and chlorine. It is also present in some coals and can be recovered from their combustion.
The purification of germanium from the aforementioned sources includes distillation of the chloride, followed by hydrolysis to the oxide, which is reduced by hydrogen to the metal. Zone-melting is used for final purification, and single semiconductor crystals are made by vaporization of germanium diiodide under conditions which cause dissociation and deposition of pure germanium.
It is the impurities in germanium which are the most important influences in its use in transistors. Therefore, after starting with the highest purity germanium the controlled introduction of trace impurities, which is termed doping, is performed to achieve the desired physical and electrical properties. Examples of doping include erbium oxide doped with thulium for use as laser crystals, germanium or silicon doped with boron or arsenic for use as semiconductors.
Because of the absolute necessity for a high purity source of germanium in its fields of uses perhaps a further review of such sources is appropriate.
Another source of the highest purity germanium is presently obtainable from the thermal decomposition of highly pure germane, GeH.sub.4, under variable conditions. The major disadvantage of this thermal decomposition technique is the percent conversion which rarely exceeds 70%-80%.
The production of germanium films using an ultra violet (UV) laser has been reported by R. W. Andreatta et al, in Applied Physics Letter 40(2), 183 (1982). This technique has the distinct disadvantage that germanium is, of course, opaque to UV light, thus as deposition increases, the rate of deposition of germanium decreases.
Therefore, the primary object of this invention is to provide a laser-induced chemical vapor deposition method for germanium and doped germanium films wherein an infrared band is employed to which germanium is transparent within the wavelength and frequency employed.
A further object of this invention is to provide a laser-induced chemical vapor deposition method for germanium and doped germanium films wherein the method produces germanium in high purity by employing a CO.sub.2 laser as the source of radiation for the production of polycrystalline germanium and doped germanium films.